This invention relates to an apparatus and method for production of high purity argon. More specifically, this invention relates to an apparatus and method for improving argon recovery from air using a cryogenic rectification column in combination with a pressure swing adsorber.
A crude argon stream containing about 0.2-15% by volume oxygen and about 1% by volume nitrogen may be recovered in the operation of a cryogenic air separation plant that produces oxygen and/or nitrogen. One of the methods generally employed to further purify the crude argon stream, is the so-called deoxo method, whereby oxygen in the crude argon stream is reacted with excess hydrogen. This process is energy intensive, since the gas stream is heated to a high reaction temperature and later cooled to cryogenic temperatures to remove the excess hydrogen and residual nitrogen. In addition, this process may not be practical in those parts of the world where hydrogen availability is limited. Another method, based on cryogenic distillation only, requires the use of a large or superstaged argon column due to the small difference in the relative volatility between argon and oxygen. Additional methods include the use of solid electrolyte membrane(s), two or more adsorption beds in a vacuum pressure swing adsorption (VPSA) process, getter/deoxo system, and temperature swing adsorption (TSA).
The cryogenic rectification of air to produce oxygen, nitrogen and/or argon is well-known. Typically, a three stage cryogenic process is used, wherein feed air is separated into nitrogen and oxygen in a double column system that uses nitrogen top vapor from a higher pressure column to reboil oxygen-rich bottom liquid in a lower pressure column, and argon-containing fluid from the lower pressure column is passed into a crude argon column for the production of argon product. For example, U.S. Pat. No. 5,440,884 by Bonequist and Lockett, disclosed a three stage cryogenic rectification system, employing a double column system with an associated crude argon column, to produce high purity (>99.999%) argon. In order to produce high purity argon, a large or superstaged argon column was used. According to U.S. Pat. No. 5,440,884, the large crude argon column is preferably divided into two separate argon columns, and a stripping column is used upstream of the double main column to suppress the thermodynamic irreversibility of the crude argon column top condenser and the lower pressure column.
U.S. Pat. No. 4,477,265 to Kumar et al., discloses the adsorption of oxygen and nitrogen from an argon-rich feed taken from the rectification column of a cryogenic air separation plant. According to this patent, argon of high purity is separated and recovered from a crude argon stream containing minor amounts of oxygen and nitrogen, by selective adsorption of these contaminants in a series of adsorption columns (beds). In an embodiment, the system utilizes two separate adsorbent columns in series wherein the first column contains a nitrogen equilibrium selective adsorbent (e.g. zeolite) that is used for nitrogen removal, and the second bed containing an oxygen rate selective adsorbent (e.g. carbon molecular sieve) used for oxygen removal. Further purification of the recovered argon may be carried out by catalytic hydrogenation of residual oxygen therein.
U.S. Pat. No. 6,527,831 to Baksh et al., discloses a vacuum pressure swing adsorption system for purifying argon from a crude argon feed stream utilizing two adsorption beds and continuously promotes the crude argon feed stream to the bed during the process with simultaneous equalization of pressure in the two beds in top-to-top end and bottom-to-bottom end equalizations in each bed following purging of each bed.
U.S. Pat. No. 6,351,971 to Nguyen et al., discloses a process and system for producing a high purity argon product with high argon recovery from an air feed stream utilizing a low ratio argon column, a high ratio argon column and a vacuum pressure swing adsorption unit in combination with a cryogenic air separation plant.
Other patents related to argon production include U.S. Pat. No. 5,730,003 to Nguyen et al., U.S. Pat. No. 5,557,951 to Prasad et al., U.S. Pat. No. 5,601,634 to Jain et al., U.S. Pat. No. 5,159,816 to Kovak et al., U.S. Pat. No. 4,239,509 to Bligh et al.